      REAL FUNCTION SIGMA(E)
C     Interchange Reactions
C     H_2^++H_2->H_3^++H
C     CROSS SECTION(CM^2)  :SIGMA
C     COLLISION ENERGY(KEV):E
C     JAERI-Data/Code 96-024(1996)
      REAL A(14)
      DATA EMIN,EMAX /1.0E-5,7.0E-3/
      DATA A /5.738E+00,-1.576E-01,1.247E-03,1.843E+00,3.101E-03,
     >        5.619E+00,1.196E+00,7.828E-02,6*0.0/
      DATA SIGMA0,ER /1.E-16,50.0/
C---+----1----+----2----+----3----+----4----+----5----+----6----+----7--
      F3(X)=SIGMA0*A(1)*(X/ER)**A(2)/(1.+(X/A(3))**(2.*A(2)))
      F4(X)=SIGMA0*A(1)*(X/ER)**A(2)/(1.+(X/A(3))**(A(2)+A(4)))
      F6(X,ALPHA)=SIGMA0*A(1)*(X/ER)**A(2)/
     >            (1.+(X/A(3))**(A(2)+A(4))+(X/A(5))**(A(2)+ALPHA))
      F11(X,ALPHA,BETA)=SIGMA0*A(1)*(X/ER)**ALPHA/
     >            (1.+(X/A(3))**(ALPHA+BETA)+(X/A(5))**(ALPHA+A(6)))
      F12(X,ALPHA,BETA)=F11(X,ALPHA,BETA)
      F13(X,ALPHA,BETA)=SIGMA0*A(1)*(X/ER)**ALPHA/
     >                  (1.+(X/A(3))**(ALPHA+BETA))
C
      SIGMA=0.0
      IF(E.LT.EMIN .OR. E.GT.EMAX) GO TO 9999
      SIGMA=F6(E,A(6))+A(7)*F6(E/A(8),A(6))
 9999 RETURN
      END
